III-V materials are useful in fabricating semiconductor devices such as heterostructure lasers, light emitting diodes, photovoltaic cells, and the like. In n-type III-V compound semiconductor wafers, a p-type conductivity modifier (e.g. zinc, cadmium, beryllium, or magnesium) is diffused into one wafer face in the presence of an inert ambient carrier gas to form a thin surface region of opposite conductivity, see, for example, Elie U.S. Pat. No. 3,264,707 and Fan et al. U.S. Pat. No. 4,547,622. Zinc in high concentration is frequently used to provide a highly doped contact layer which is an important ingredient for achieving a good ohmic contact with low contact resistance.
Zinc diffusion with high zinc concentrations is extensively used in disordering of GaAs/AlGaAs quantum wells to fabricate low threshold lasers. The more expensive epitaxy process to form the junction is replaced by zinc diffusion to form GaSb photoactive transducers as disclosed in Fraas et al. application Ser. No. 07/523,710 filed May 14, 1990, now Pat. No. 5,091,018. Attention is directed to that patent and the prior art cited therein as background prior art applicable to this invention.
Conventionally, zinc doping of III-V compound semiconductors is carried out by annealing the compound semiconductor at high temperatures in a "pseudo closed box" system. In the case of GaAs, a typical anneal would be at 600.degree. C. under zinc arsenide and hydrogen ambient as zinc arsenide provides the zinc source and the needed arsenic overpressure to prevent decomposition of the GaAs surface. In the case of GaSb, elemental zinc and elemental antimony have been used to provide zinc diffusion without decomposition of the GaSb surface. Temperature and time determine the surface concentration of the zinc and depth of the emitter junction.
Prior GaAs diffusion techniques provide dopant concentrations well in excess of 10.sup.19 /cm.sup.3. We have found that a lighter dopant concentration is needed for high efficiency solar cells. Dopant concentrations less than about 10.sup.19 /cm.sup.3 are required to preserve minority carrier lifetimes in the solar cell zinc doped layer. Minority carrier lifetimes affect light generated carrier collection and hence short circuit currents.
In our earlier GaSb process, emitter etching has been used to provide a patterned diffusion that gives high energy conversion efficiency. A problem with this process is that cell costs are high because the process cannot be easily scaled up for mass production.